One important characteristic of an ink for ink-jet recording is good ozone resistance. This means to prevent discoloration (degradation of image quality) of printed matter by ink-jet recording attributable to contact with the ozone in air. Copper phthalocyanine dyes, which often are used as colorants for cyan colored inks, have good light fastness as compared with dyes typically used for magenta and yellow colors. However, many copper phthalocyanine dyes have inadequate ozone resistance. Some copper phthalocyanine pigments have been developed which have good light fastness as well as ozone resistance.
Another important characteristic of an ink for ink-jet recording is resistance to mildewing. When mildew is generated, it becomes a foreign matter, so that ejecting stability at the nozzle of ink-jet head is decreased and the nozzle can become clogged. Accordingly, a thiazole compound can be added to the ink as a mildew proofing agent whether or not a copper phthalocyanine pigment is used as a colorant.
Although generation of mildew can be restrained in the ink to which a thiazole compound as a mildew proofing agent is added, another problem arises with generation of precipitates. This tendency is especially conspicuous when a copper phthalocyanine pigment is used as a colorant. Further, an anticorrosive agent such as a benzotriazole compound is sometimes additionally compounded to the ink for the prevention of corrosion of metallic materials that are brought into contact with the ink in the ink passage of ink-jet head. The anticorrosive agent can further contribute to the generation of precipitates.